Isotropic (Single Refracting) & Anisotropic (Doubly Refracting)

Isotropic or Single Refracting (S.R.): A ray of light which enters an Isotropic substance will obey Snell's lawof refraction and will produce a single refracted ray, regardless of the direction in which the ray enters the gem.

They are all defined by a single refractive index, since light is propagated with equal velocity in all directions.

The index of refraction of such optically isotropic crystals does not depend on the direction in which the ray of light passes through them.

Anisotropic or Doubly Refracting (D.R.): A ray of light which enters an anisotropic substance is not only bent or refracted, but it is also broken or split into two rays, each of which travels with a different velocity and hence has a different Refractive Index.

Each of these rays is polarised and differs from the other in velocity and vibration direction. Their vibration planes are perpendicular to each other.

The light ray with a constant velocity of propagation in different directions is called the ordinary ray. The R.I. of the ordinary ray is constant i.e. it does not vary with direction of propagation.

The rays of variable velocity of propagation of light in different directions are called the extra-ordinary rays. The R.I. of the extra-ordinary ray varies not only with direction of propagation but also with planes of polarization.

Gemstones belonging to the tetragonal, hexagonal and trigonal systems have two principal R.I.'s - Omega for ordinary ray and Epsilon for extra-ordinary ray.

Gemstones belonging to orthorhombic, monoclinic and triclinic systems have three principal R.I.'s - Alpha (least) for the fastest ray, Beta (intermediate) and Gamma (greatest) for the slowest ray.

In addition, every day ray of light is differently refracted for different colours of light.